Parallel batteries and a question regarding reserve capacity

Actually paralleling two batteries will gain you more than double. The higher ratio of amp draw relative to the amp-hr rating, the less amp-hr are actually available...by doubling the batteries, you cut that ratio. Don't expect a huge improvement but a little.

6V golf cart batteries are popular because it's hard to find true deep cycle 12v batteries and they often come at a premium price. 200-300amps out of golf cart isn't going to harm it.